The statement said either during or immediately following someones attempt to make an extremely valid point that makes the person stop talking and listen to your potentially much less important and usually unrelated opinion. The best setup to a conversational punchline. Ever.

S- "I've never told anyone this before C, but I think I have an authority fettish because of my corrupted relationships to male figureheads in my childhood."
C- "Here's the thing -I'm hungry. Buy me a McDouble."

S- "...So then the condom broke and-"
C- " Here's the thing -I don't have the funds or the reputation to be a godfather to some bastards bastard child. Buy me a McDouble."